OK guys, I've been playing around and think I've found something. Give this a go:
1. Create a new project.
2. Make 3 folders.
3. Record a handful of tracks (audio or MIDI). Move these tracks into the folders so you have at least 1 or more track in each folder, and have 3 tracks alone outside of the folders.
4. Move the non-foldered tracks to the top of the track view so you have the 3 tracks, then 3 folders with tracks inside them underneath. Make sure each folder is CLOSED.
5. Z-Drag over the top of any of the non-foldered tracks at the top. It'll work correctly no matter what you do - be it single tracks or multiple tracks.
6. Now move the non-foldered tracks to the bottom of the track view so the folders are first. Z-Drag on any or all of those tracks and... zoom is screwy. Sometimes you'll get no zoom, sometimes if you scroll up you'll find the first one is maximised but the others aren't.
7. Open the first folder, Z-Drag across parts of it. It'll work correctly. Close the first folder.
8. Open the second folder. Z-Drag across parts of it. Same zoom issues as the non-foldered tracks when they were moved to the bottom. This will also happen with the third folder too.
9. Now open all of the folders. Z-Drag works correctly on the entire project.
From what I've been able to work out, zoom works correctly for every track until it encounters a closed folder and then every track after that does not zoom correctly.
Hopefully this isn't just a screwy project of mine and can be reproduced so it can be fixed. :)